In order to preserve the transmission qualities of a communications cable, it becomes necessary to prevent the ingress of moisture into a multiconductor core of the cable. This may be accomplished by introducing a pressurized gas. In an increasingly popular technique, the cable core is filled with a waterproofing material after which a metallic shield having its major surfaces flooded with a wafterproofing material is wrapped about the core. The last-described technique produces what is referred to as a filled cable and avoids the necessity of pressurizing the cable.
For some filled cables, a dual metallic shield system is used. An inner shield which is made of a material having a relatively high electrical conductivity is used to dissipate stray electromagnetic currents and lightning, for example. Formed about the inner shield which may comprise aluminum, for example, is an outer shield which is made of a material such as steel having a relatively high modulus of elasticity. The outer shield provides suitable mechanical strength for the cable. Such a cable which also includes a plastic jacket is commonly referred to as an ASP cable.
It is not uncommon to bond the plastic outer jacket to the outer surface of the outer shield. By bonding the plastic jacket to the outer shield, which generally is corrugated, it has been found that the resistance of the cable to moisture diffusion is increased substantially. Further, if the jacket is not bonded to an adjacent shield, the pulling of the cable into an underground duct more often than not will cause a separation of the jacekt from the shield.
In the manufacture of a bonded sheath cable, a metallic tape such as a steel tape is precoated on one or both surfaces with a layer of an adhesive polymer material. Then the tape is wrapped about an inner shield which has been flooded with a waterproofing material and the core to form an outer shield having a longitudinal overlapped seam of which an overlying portion is directed inwardly toward the core. When a plastic jacket is extruded over the shielded core, the heat from the semimolten plastic material causes the adhesive layer on the outer surface of the outer shield to bond the jacket to the shield. This provides a laminate which reduces substantially the ingress of moisture into the core. Also, it provides mechanical strength to resist buckling, crushing and scuffing.
There also is a need to seal the longitudinal overlapped seam of the outer shield. This is necessary for several reasons. The sealing of the seam further reduces the probability of moisture ingress. Secondly, it prevents the escape of waterproofing material from the interface between the inner and outer shields to the external surface of the outer shield. Should any such material become deposited on the outer surface of the outer shield, bonding of the jacket to the shield is impaired. As a result, the jacket may exhibit irregularities or become easily separated from the outer shield when the cable is pulled into a duct. Further, a sealed seam is helpful in preventing the overlying edge portion of the outer shield from protruding into the jacket and weakening the plastic.
In the prior art, the sealing of the longitudinal seam has been accomplished in several ways. For example, inasmuch as longitudinal edge surfaces of the tape from which the outer shield is formed have been precoated with an adhesive material, heat of the extrusion causes a bonding of the overlapped longitudinal edge portion to occur. In another technique, the seam is formed, then opened and filled with a bead of adhesive material. See U.S. Pat. No. 4,035,211 which issued on July 12, 1977 in the names of R. G. Bill et al.
The above-mentioned techniques are not entirely satisfactory for sealing the longitudinal seam of a steel shield and for preventing water flow between the inner and outer shields. To reopen the seam after it has been formed may weaken the edge portions and the reclosing may not provide an altogether acceptable configuration. Further, the forming of the longitudinal seam of the steel shield with the overlying portion being directed inwardly results in cavities being formed between the inner and outer shields. These cavities are not filled with a waterproofing material inasmuch as limited quantities of that material are applied to the surfaces of the shields to avoid its escape through the seam. In fact, only enough waterproofing material is applied to the inner shield to fill the corrugations. The use of a precoated adhesive material or a bead in the seam is not sufficient to fill these cavities and eliminate potential conduits through which moisture may travel.
What is needed and what seemingly is not provided by the prior art is a bonded ASP cable having a reliably sealed overlapped seam in the outermost shield. The sealing of the seam should be such that any cavities between shields or between a single shield and adjacent portions of the cable are filled. Desirably, the sealing is accomplished in a manner which does not require a restructuring of the seam.